The present disclosure relates to a detecting device, a power receiving device, a contactless power transmission system, and a detecting method to detect the existence of a conductor such as a metal.
In recent years, the contactless power transmission system to supply power in a contactless manner (wireless power feed) is being actively developed. As the system to realize the wireless power feed, two kinds of techniques exist in a rough classification.
One is the electromagnetic induction system, already widely known. In the electromagnetic induction system, the degree of coupling between the power transmission side and the power reception side is very high and power feed with high efficiency is possible. However, the coupling coefficient between the power transmission side and the power reception side needs to be kept high. Therefore, the power transmission efficiency between the coils of the power transmission side and the power reception side (hereinafter, referred to as the “inter-coil efficiency”) greatly deteriorates when the distance between the power transmission side and the power reception side is long or when there is positional misalignment.
The other is the technique called the magnetic resonance system. It has a characteristic that the resonance phenomenon is aggressively utilized and thereby even little magnetic flux is permitted as the magnetic flux shared by the power feed source and the power feed target. In the magnetic resonance system, even when the coupling coefficient is small, the inter-coil efficiency does not deteriorate if the quality factor (Q-factor) is high. The Q-factor is an index representing the relationship between retention and loss of energy in a circuit having the coil of the power transmission side or the power reception side (indicating the intensity of the resonance of a resonant circuit). That is, axis alignment between the transmission-side coil and the reception-side coil is unnecessary and flexibility in the position and distance of the power transmission side and the power reception side is high.
One of the important factors in the contactless power transmission system is countermeasures against heat generation of a metal foreign substance. Whether the system is the electromagnetic induction system or the magnetic resonance system, in contactless power feed, if a metal exists between the power transmission side and the power reception side, possibly an eddy current is generated in the metal and heat generation of the metal is caused. As countermeasures against this heat generation, a large number of techniques to detect the metal foreign substance have been proposed. For example, techniques using an optical sensor or a temperature sensor are known. However, in the detecting method using a sensor, the cost is high when the power feed range is wide like in the magnetic resonance system. Furthermore, for example, in the case of the temperature sensor, the output result of the temperature sensor depends on the thermal conductivity of the environments around it, which imposes design restrictions on the pieces of apparatus of the power transmission side and the power reception side.
To address this, the following technique has been proposed. Specifically, whether or not a metal foreign substance is present is determined by monitoring change in a parameter (current, voltage, etc.) when the metal foreign substance encroaches between the power transmission side and the power reception side. Such a technique can suppress the cost without the necessity to impose design restrictions and so forth. For example, Japanese Patent Laid-open No. 2008-206231 (Patent Document 1) proposes a method in which a metal foreign substance is detected based on the degree of modulation (information on change in the amplitude and phase) in the communication between the power transmission side and the power reception side. Furthermore, Japanese Patent Laid-open No. 2001-275280 (Patent Document 2) proposes a method in which a metal foreign substance is detected based on the eddy current loss (foreign substance detection based on the DC (direct current)-DC efficiency).